The present invention relates to cryostat systems in the field of NMR spectroscopy and related experimental fields of application but not limited thereto, and in particular to high-field NMR systems having pulse tube coolers operating in the sub-helium temperature range.
This type of spectroscopy is presently one of the most exact analytic methods when looking at highly complex chemical and biological molecules and makes it possible to reveal a variety of findings which have not been possible up to now.
More specifically, the present invention relates to the cryostat of NMR systems, and in particular aims directly at high-field NMR applications. The most recent commercially available high-field magnet systems show a proton resonance frequency V.sub.R of about 750 MHz, which corresponds to a field of about 17.63 Tesla. For certain applications higher fields are required, e.g. 900 MHz which corresponds to 21.1 T and even higher fields to reach the GigaHertz region. Spectroscopy requires high field strength and low field drift, usually in the region of 10.sup.-8 per hour or less of the central field strength. Both of these may be achieved by using present-day standard Nb.sub.3 Sn or NbTi wires or tapes, in combination with HTC wire technology for 20 Tesla systems, as proposed by Komarek in Hochstromanwendung der Supraleitung, High-power applications in Superconductivity, Teubner Studienbucher, page 93 and 94, and by sub-cooling the helium bath in which the magnet is immersed. This sub-cooling of the helium bath is done by external means, most often by pumping the bath down to the required temperature by means of a pump assembly. The definition used herein for sub-cooling, actually refers to temperatures in the range below 4.2 K, in particular around the lambda transition point and down to 1.8 K in case large pumps are being used. Because of the low vapour pressure and film flow of helium any further reduction in temperature is difficult to achieve. At a saturated vapour pressure of about 50 mbar a transition occurs at T.lambda.=2.172 K which is termed the lambda point. At this point, liquid helium I and liquid helium II are separated by the common boundary phase, called the lambda line. It is also a well known fact that when pumping the system down to 2.172 K helium creep may be detected in the vicinity of the heat exchanging surface due to the change in physical properties of liquid helium as a result of reducing the vapour pressure.
It is well known in high-field NMR systems that pumping down a helium bath uses both open and closed loop control systems technology attached to the cryostat. Pumping down means reducing the overall pressure of the helium bath down to 60 mbar or lower.
A typical high-field system is described in GB patent number 2286450A.
In order to run the system unattended, the control of the pump assembly maintaining the sub-cooled temperature in the helium bath as well as the feedback control of the flow rate of the pumped helium gas controlling the helium bath temperature must be monitored continuously, which in turn means additional costs for investment and maintenance. If the pumping system fails, the helium bath and magnet will begin warming towards 4.2 K, and the magnet will quench. In order to prevent this happening, and ensure continuous operation, equipment redundancy is required which also means additional costs. A further shortcoming of such a pumped system could be the pipe work of the pumping system connected to the cryostat, as this could be a permanent source of vibration transmitted to the internal components of the cryostat, e.g. the radiation shields and other parts receptive of vibration. A further shortcoming of such a pumped system could be the increased penetration of ice into the system due to the under pressure. This could cause severe problems as ice could gradually build up within the turrets, e.g. starting to build up at electrical connections routed from the coil up to the tube inlet and into the neck tube, and virtually block the neck tubes without the user's knowledge. Thus, a pumped system also needs permanent inspection, electronic monitoring and maintenance.
Reducing the gas pressure in the vessel housing the magnet sections also means that a control mechanism has to be introduced to the 2.2 K stage. This control mechanism usually is a special valve, probably a needle valve with which extremely low flow rates can be achieved. Because of the small flow rate, which in fact is termed `leakage flow rate`, this leakage flow has to be controlled by setting it from the accessible warm end of the valve spindle. Care has to be taken so that ice will not penetrate into the system. Due to the under pressure, i.e. suction process within the valve system, provision has to be made to safeguard against particles penetrating the valve seat which would make it impossible to set the desired flow rate. Also as has been mentioned above, this system could be subject to icing problems which are most likely to occur during warm-up and cool-down or when ice would enter from the top vessel, which in turn makes it difficult, if not impossible to adjust the flow rate. Furthermore, this type of precision control elements are expensive and add to the overall costs of the NMR system.
In order to achieve a stable temperature at the lambda temperature level, approximately 40% of the enthalpy of the liquid helium has to be withdrawn out of the low helium reservoir and has to be added to the total boil-off in a pumped system. Hence, a pumped system inherently shows an increased boil-off of liquid helium and added overall running and maintenance costs which are higher than with 4.2 K dewars of comparable size.
In summary, a sub-cooled system presents a challenge in that the overall structure and layout is more complicated and subject to failure than cryostats operating at the normal boiling point of liquid helium.
It has not previously been feasible to introduce a piston-driven cryo-cooler into a NMR system, because of the induced vibration to the overall system, to the magnet which makes it impossible to attain a good NMR signal without distortion and also because of the non-availability of low-temperature cryo-coolers in general for this temperature region.
Meanwhile, the current technology in cryo-coolers has considerably advanced and it is possible to achieve temperatures as low as 2.13 K even with piston-less systems. This would mean, as has been emphasised above, that this temperature region could be achieved without pumping and the bath actually behaves just like any other 4.2 K system.
A cooler which gives a cooling capacity at lambda temperatures will be called a lambda cooler in the following description.
Therefore, the aim of the present invention is to provide a non-pumped mechanical cooling system to a high-field NMR system.
According to the present invention there is provided a cryostat system comprising means defining first and second volumes of cooling liquid, a superconducting magnetic coil structure immersed within one of said volumes of cooling liquid, and cooling means for maintaining an operating temperature of the coil structure in the sub-helium temperature range, characterised in that said cooling means is a pulse tube refrigerator which extends into said first and second volumes of cooling liquid.
The pulse tube refrigerator includes a cold end and a heat exchanger connected thereto which extends into the volume of liquid in which the coil structure is immersed.
The pulse tube refrigerator can conveniently be incorporated in an existing neck tube to reduce the boil-off of cooling liquid.
The warm end of the pulse tube refrigerator may be pre-cooled by either a further pulse tube refrigerator operating at 80 K, or by being directly thermally linked to the liquid nitrogen temperature level and/or at the lower temperature of the radiation shields at the internal linking position in the turnet.
The pulse tube refrigerator may also be used to support and cool the radiation shields.
The pulse tube refrigerator, if it is designed to be rigid, may also be used both to support and cool the radiation shields in the case of a multistage cooler, and simultaneously to support the neck tube, thus suspending the magnet system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.